Add metrics aggregation tests to ensure correctness

[cijothomas]

The current tests are only covering Counters.

We need to replicate the same for all instruments. Multi-threaded tests are nice to have, but not a must before beta.

[utpilla]

I would like to take this up.

[cijothomas]

Thanks! Assigned. Lack of test is giving me less confidence to do big refactoring to get perf boost. For now I'll focus on Counters, which do have some coverage.

[cijothomas]

Closing as we have this mostly covered.